The characteristics required for such materials include the possibility of being used at temperatures of 1200.degree. to 1500.degree. C., the lowest possible density and thermal conductivity and the highest possible compressive strength, bending strength, tensile strength, resistance to thermal shocks through heating and cooling, resistance to strong acoustic vibrations and other characteristics.
A first generation of re-usable heat insulating materials comprised fibrous, slightly sintered systems such as the systems of the Lockheed Insulation (LI) type bearing the trade names LI-900, LI-1500 and LI-2200 and having densities of 144 kg/m.sup.3 (0.144 g/cm.sup.3), 240 kg/m.sup.3 (0.240 g/cm.sup.3) and 352 kg/m.sup.3 (0.352 g/cm.sup.3), respectively, which materials are formed of silica fibers.
According to the known method, these materials were manufactured by forming, in a V-blender, a slurry comprising silica fibers having a diameter of 1 to 3 .mu.m and water purified on ion exchange resins, in the presence of starch, ammonia and a surface-active agent, by then pouring this slurry into a rectangular mold with perforated bottom and side walls, and then slightly compressing the slurry in the mold with partial removal of water. The green billet thus obtained was then dried at temperatures of up to 105.degree. C. (220.degree. F.), fired at temperatures of up to 1260.degree. C. (2300.degree. F.) and the resulting sintered billet was machined to obtain the desired final dimensions and shape.
The main disadvantages of the materials identified hereabove were: considerable shrinkage representing nearly 25% (linear shrinkage) after 16 hours at 1250.degree. C., related to the low thermal resistance of the fibers and strength or tenacity not exceeding 0.08 MPa under tension in the "weak" direction for a material with a density of 144 kg/m.sup.3, because of the poor sintering of the fibers into a skeleton.
A second generation of the materials in question, known as FRCI (Fibrous Refractory Composite Insulation) did not have such drawbacks. Their strength was improved by intensification of the sintering process using easily fusible boron-containing additives. In addition, the inevitable tendency in this process to additional shrinkage was compensated for or neutralized by the introduction into the system of more refractory aluminoborosilicate fibers of the "Nextel.RTM.-312" type. Now, the main basis of the process was the same as in the process for manufacturing LI type materials, except that the larger diameter (11 .mu.m) of the twisted "Nextel.RTM." fiber introduced impeded its dispersion and that it was difficult to produce the material with a homogeneous structure.
The third and last generation of materials of the type in question is formed by the so-called HTP (High Thermal Performance) and AETB (Alumina Enhanced Thermal Barrier) materials.
The HTP materials of different kinds and densities include the combination of silica fibers and alumina fibers having a diameter of 3 .mu.m.
The AETB material is a three-component composition comprising silica fibers, 3 .mu.m aluminoborosilicate fibers and 3 .mu.m alumina fibers. In either case, the sintering of the fibers to form the skeleton is carried out thanks to the presence of easily fusible boron anhydride which is formed from boron or its compounds upon firing.
Owing to the fact that the geometrical parameters of the silica fibers and the ceramic fibers were brought closer together, the materials of the third generation ensured more homogeneous mixing of the ingredients, uniformity of the macro- and micro-structures and less dissipation of properties.
Despite their positive characteristics, the materials of the second and third generations have a common disadvantage, i.e. that it is difficult to combine the ingredients of a 2 or 3-component system, which leads to non-homogeneous structure and properties. There also is the added risk of high internal stresses owing to the fact that the thermal expansion coefficients and other properties of the silica fibers and of the ceramic fibers are different. The fact that the "Nextel.RTM." and "Saffil.RTM." type fibers used are far more costly than silica fibers and that, consequently, the price of the final product increases substantially is also important.